Multi-pass heat exchanger

ABSTRACT

A multi-pass heat exchanger is provided. The multi-pass heat exchanger may be used in a fuel processing subsystem for cooling a reformate flow or in any other application where uniform outlet temperatures are desired. The multi-pass heat exchanger includes a plurality of tube run groups having a first pass, an intermediate pass and a final pass. The respective locations of the passes relative to each other provide more uniform outlet temperatures than conventional automotive-style, parallel flow, single-pass heat exchangers.

FIELD OF THE INVENTION

This invention relates to heat exchangers, and in more particularapplications, to multi-path heat exchangers wherein at least one fluidrequires a relatively uniform temperature as it exits the heatexchanger, such as heat exchangers for a reformate flow in a fuelprocessing subsystem for a fuel cell system.

BACKGROUND OF THE INVENTION

In many proton exchange membrane (PEM) fuel cell systems, a fuel such asmethane or a similar hydrocarbon fuel is converted into a hydrogen-richstream for the anode side of the fuel cell. In many systems, humidifiednatural gas (methane) and air are chemically converted to ahydrogen-rich stream known as reformate by a fuel processing subsystemof the fuel cell system. This conversion takes place in a reformer wherethe hydrogen is catalytically released from the hydrocarbon fuel. Acommon type of reformer is an Auto-Thermal Reformer (ATR), which usesair and steam as oxidizing reactants. As the hydrogen is liberated, asubstantial amount of carbon monoxide (CO) is created which must bereduced to a low level (typically less than 10 ppm) to prevent poisoningof the PEM membrane.

The catalytic reforming process consists of an oxygenolysis reactionwith an associated water-gas shift [CH₄+H₂O

CO+3H₂, CO+H₂O

CO₂+H₂] and/or a partial oxidation reaction [CH₄+½O₂

CO+2H₂]. While the water-gas shift reaction removes some of the CO fromthe reformats flow stream, the overall reformate stream will alwayscontain some level of CO, the amount being dependent upon thetemperature at which the reforming process occurs. After the initialreactions, the CO level of the reformate flow is well above theacceptable level for the PEM fuel cell. To reduce the CO concentrationto within acceptable levels, several catalytic reactions will generallybe used in the fuel processing subsystem to remove CO in the reformateflow. Typical reactions for reduction of CO in the reformate flowinclude the aforementioned water-gas shift, as well as a selectiveoxidation reaction over a precious metal catalyst (with a small amountof air added to the reformate stream to provide oxygen). Generally,several stages of CO cleanup are required to obtain a reformate streamwith an acceptable CO level. Each of the stages of CO cleanup requiresthe reformate temperature be reduced to precise temperature ranges sothat the desired catalytic reactions will occur and the loading amountof precious metal catalyst can be minimized.

In this regard, liquid-cooled heat exchangers are frequently employed tocontrol the reformate temperature at each stage because of their compactsize when compared to gas-cooled heat exchangers. Further, becauseliquid water entering the fuel processing subsystem must be heated sothat it can be converted to steam for the reforming reactions, it isthermally efficient to use process water as the liquid coolant for theheat exchangers to cool the reformate flow prior to CO removal. However,such an approach can be difficult to implement.

Specifically, it would be economical to leverage automotive-style heatexchangers to be utilized as heat exchangers for fuel processingsubsystems. However, these heat exchangers can have certain drawbacks.For example, in typical parallel flow single-pass automotive-style heatexchangers, the flow that is being cooled typically will have localizedcool regions because of the subcooled inlet side of the heat exchangerwhere the coolant or refrigerant enters. Additionally, if the coolant iscompletely vaporized prior to exiting the heat exchanger, the flow beingcooled will have localized hot regions. These phenomenon produce atemperature gradient across the exhaust face of the heat exchanger inthe flow being cooled. Such temperature gradients can be unacceptable infuel processing subsystems, which typically require a uniformtemperature in the reformate flow exiting a heat exchanger. The variancebetween the localized hot and cool regions can have significant negativeeffects on the CO removal processes within fuel processing subsystemssuch as decreased efficiency and decreased life of the catalyst.

SUMMARY OF THE INVENTION

In accordance with one form of the invention, a heat exchanger isprovided for transferring heat between a first fluid flow and a secondfluid flow. The heat exchanger includes an inlet manifold, an outletmanifold, and a plurality of aligned and spaced tube run groups. Thetube runs groups extend between the inlet manifold and the outletmanifold to direct the first fluid flow through the heat exchanger, eachtube run group having three tube runs. The tube runs of each groupinclude a first tube run coupled to the inlet manifold to direct thefirst fluid in a first direction, an intermediate tube run coupled tothe first tube run to receive the first fluid therefrom and direct thefirst fluid in a second direction opposite the first direction, and afinal tube run coupled to the intermediate tube run to receive the firstfluid therefrom and direct the first fluid in the first direction to theoutlet manifold, the intermediate tube run located adjacent the firsttube run and the final tube run. For each adjacent pair of tube rungroups, the final tube run of one of the tube run groups of the pairbeing located adjacent the first tube run of the other tube run group ofthe pair. The heat exchanger also includes a plurality of fins extendingbetween the adjacent tube runs of the tube groups from an inlet face toan outlet face of the heat exchanger.

In one form, the tube runs are flattened tubes.

In accordance with one form, the fins are serpentine fins.

In one form, the tube runs are constructed of aluminum.

In a preferred form, each tube run group comprises a single tube thatdefines the three tube runs.

According to one form, each single tube is arranged in a generallyserpentine configuration.

According to one form, a method is provided for transferring heat from afirst fluid to a second fluid in a heat exchanger.

In accordance with one form, the method includes the steps of:

flowing the first fluid from the inlet to the outlet via a plurality ofaligned multi-pass flow paths;

each of the flow paths including a first pass extending between thefirst and second sides in a first flow direction transverse to the inletand outlet faces, an intermediate pass extending between the first andsecond sides in a flow direction opposite the first direction transverseto the inlet and outlet faces, and a final pass extending between thefirst and second sides in the first flow direction, the passes beingparallel to each other;

each of the intermediate passes running between the first and finalpasses of the associated flow path;

for each adjacent pair of flow paths, the final pass of one of the flowpaths of the pair running adjacent the first pass of the other flow pathof the pair;

flowing the second fluid from the first face to the second face via flowpaths between and transverse to the passes; and

transferring heat between the first and second fluids as the first andsecond fluids flow through the respective flow paths.

In one form, the first fluid comprises water.

In a preferred form, the method further includes the step oftransferring sufficient heat to only partially vaporize the first fluid.

Other objects, advantages, and features will become apparent from acomplete review of the entire specification, including the appendedclaims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a fuel processing subsystemincluding heat exchangers embodying the present invention;

FIG. 2 is a perspective view of a heat exchanger embodying the presentinvention;

FIG. 3 is a somewhat diagrammatic representation of a portion of theheat exchanger of FIG. 2; and

FIG. 4 is graph depicting a comparison of the temperature profiles of areformate flow flowing through a heat exchanger embodying the presentinvention and a single-pass cross-flow type heat exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is susceptible of embodiment in manydifferent forms, there are shown in the drawings and will be describedherein in detail specific embodiments thereof with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the specific embodiments illustrated.

As seen in FIG. 1, a pair of heat exchangers 10 embodying the presentinvention are provided for use in a fuel processing subsystem, shownschematically at 12, for producing a reformate flow 14 from ahydrocarbon flow 16 and for reducing a level of carbon monoxide (CO) inthe reformate flow 14 for use in a proton exchange membrane fuel cellsystem (not shown). As used in the specification and claims, the phrasefuel flow is meant to encompass both the hydrocarbon flow 16 and thereformate flow 14. While two of the heat exchangers 10 are shown, itshould be understood that the heat exchangers 10 do not depend on eachother and can operate independently. Additionally, any number of heatexchangers 10 can be utilized as required by the fuel processingsubsystem 12. For example, some subsystems 12 may require a single heatexchanger 10, while others may require three or more of the heatexchangers 10. Each of the heat exchangers 10 provides an advantageouscoolant flow scheme that can allow for optimization of the temperaturedistribution of the reformate flow 14 exiting the heat exchanger 10.

In the illustrated embodiment, the fuel processing subsystem 12 includesa reformer 18. A commonly used method called auto-thermal reforming maybe used to produce the reformate flow 14 from the hydrocarbon flow 16 inthe reformer 18. The reactions consist of an oxygenolysis reaction, apartial oxidation, and a water-gas shift [CH₄+H₂O

CO+3H₂, CH₄+½O₂

CO+2H₂, CO+H₂O

CO₂+H₂]. For these catalytic reactions to occur, the reactants must bebrought to an elevated temperature typically in excess of 500° C. Asshown in the first reaction, a process water flow 20 is used in the formof superheated steam 22 to partially elevate the temperatures of thereactants entering the reformer 18. As in most fuel processingsubsystems for fuel cell systems, the necessary heat to create the steamflow 22 must be added to the process water flow 20 from an externalsource such as a heater or, as shown in FIG. 1, by burning a reformate,hydrogen, natural gas, or other hydrocarbon containing combustiblemixture, such as anode tail gas stream 24 and transferring heat in aheat exchanger 26 to create the steam flow 22. In the illustratedembodiment, the process water flow 20 is supplied by a suitablepressurized water source 27.

As shown in the above mentioned reactions, CO is created in thereforming process. The CO created must be removed before entering a fuelcell because it is poisonous to the membrane, limiting the fuel cellperformance and lifetime. Additionally, the amount of CO created in thereforming reactions is highly dependent upon the reaction temperature.At higher temperatures, the reactions yield more hydrogen gas useful inthe fuel cell, but also yield more poisonous CO. In order to eliminatethe poisonous CO from the reformate flow 14, CO elimination stages maybe utilized.

As illustrated in FIG. 1, after the hydrocarbon flow 16 is used toproduce the reformate flow 14 in the reformer 18, the reformate flow 14is flowed to at least one water-gas shift 28. The water-gas shift 28 isutilized to further remove poisonous CO from the reformate flow 14 andcreate more hydrogen gas for use in the fuel cell system. The water-gasshift requires water as shown in the water-gas shift reaction [CO+H₂O

CO₂+H₂]. Optionally, additional water (as indicated by the dotted linesin FIG. 1) may be added at the water-gas shift 28 as required by thefuel processing subsystem 12 to maintain the water-gas shift reaction.The additional water may come from the process water flow 20, watersource 27, or any other suitable water source. Additionally, multiplewater-gas shifts 28 and 29 may be utilized to further reduce the amountof poisonous CO in the reformate flow 14.

Even after multiple water-gas shift units 28 and 29, the reformate flow14 still typically contains excessive amounts of poisonous CO in thereformate flow 14. To eliminate more of the poisonous CO, at least onehydrogen purification device or subsystem, such as selective oxidizer 30may be utilized. Selective oxidation reactions typically require a smallamount of air to be added to the reformate flow 14 to provide oxygen asrequired by the selective oxidation reaction [CO+½O₂

CO₂]. Selective oxidation reactions typically occur over a preciousmetal catalyst. For the catalytic reaction to occur, the reformate flow14 must be reduced to a desired temperature range to optimize theefficiency of the precious metal catalyst. Typically, selectiveoxidation occurs in a temperature range of 130° C. to 180° C. Highlyefficient selective oxidation occurs over a much narrower temperaturerange depending upon the catalyst. To minimize the amount of catalystrequired for the selective oxidation reaction, it is preferred that thetemperature to which the reformate is cooled be precisely controlled. Inthe embodiment of FIG. 1, multiple selective oxidizers 30 and 31 areutilized and operate at different desired temperature ranges from eachother to remove poisonous CO, preferably to a level less than 10 ppm inthe reformate flow 14. Each of the heat exchangers 10 is used to coolthe reformate flow 14 to within the desired temperature range for therespective selective oxidizers 30 and 31.

FIG. 2 illustrates a preferred embodiment for each of the heatexchangers 10. Each of the heat exchangers 10 includes a planar inletface 50 to receive the reformate flow 14, a planar outlet face 52opposite the inlet face 50 to exhaust the reformate flow 14, a fluidinlet 54 on a first side 56 of the faces 50, 52 to receive a first fluidor coolant, such as a water flow 53 from the source 27, and a fluidoutlet 58 on a second side 60 of the faces 50, 52 opposite the firstside 56 to exhaust the flow 53. Each heat exchanger 10 further includesan inlet manifold 100 at the first side 56 to receive and distribute theflow 53. The water flow 53 is dispersed by the inlet manifold 100 intoto a plurality of aligned and spaced tube run groups 104 for directingthe water flow 53 through a plurality of aligned multi-pass flow pathsshown schematically by the dashed and arrowed lines 106. The water flow53 flows through the tube run groups 104 while in a heat exchangerelationship with a second fluid, which is the reformate flow 14 in theillustrated embodiment, flowing through a plurality of serpentine fins108. In the illustrated embodiment, each tube run group 104 is providedin the form of a flattened multi-port tube 110 that has been shaped intoa serpentine configuration. From the tube run groups 104, the water flow53 flows to an outlet manifold 112 at the second side 60.

As best seen in FIG. 3, each tube run group 104 includes a first tuberun 120 directing the water flow 53 through a first pass 121 of the flowpath 106, an intermediate tube run 122 directing the water flow 53through an intermediate pass 123 of the flow path 106 and being coupledto the first tube run 120, and a final tube run 124 directing the waterflow 53 through a final pass 125 of the flow path 106 and being coupledto the intermediate tube run 122 and the outlet manifold 112. As shownin FIG. 2, for each tube run group 104, the intermediate tube run 122 islocated adjacent the first tube run 120 and the final tube run 124.Additionally, for each adjacent pair of tube run groups 104, the finalrun 124 of one of the tube run groups 104 of the pair is locatedadjacent the first run 120 of the other tube run group 104 of the pair.In the illustrated embodiment, the tube runs 120, 122, and 124 of eachof the tubes 110 are coupled by 180° bends 126 in the tube 110.

In each tube run group 104, the water flow 53 is directed by the firstrun 120 in a first direction, indicated by arrow A, through the firstpass 121. The water flow 53 exits the first tube run 120 and enters thebend 126 at the second side 60 prior to entering the intermediate tuberun 122. The water flow 53 is directed by the intermediate tube run 122in a second direction, indicated by arrow B, substantially opposite thefirst direction A through the intermediate pass 123 towards the firstside 56. The water flow 53 flows from the intermediate tube run 122through the bend 126 at the first side 56 into the final tube run 124.The water flow 53 is directed by the final tube run 124 from the firstside 56 through the final pass 125 in the direction A to the outletmanifold 112 at the second side 60.

While the water flow 53 is passing through the tube run groups 104, thereformate flow 14 is passing through the fins 108. The reformate flow 14flows from the inlet face 50 through the fins 108 to the outlet face 52in a direction as indicated by arrow C in FIG. 2. It should beunderstood that the reformate flow 14 could also flow in an oppositedirection as indicated by arrow D. While the fins 108 are shown asserpentine, it should be understood that the fins 108 can be of anysuitable type known in the art to provide a sufficient heat exchangerelationship between the reformate flow 14 and the water flow 53.

It should be understood that the embodiment shown in FIGS. 2 and 3 ismerely one form of the present invention. For example, while each of thetube run groups 104 is shown in the form of a single flattenedmulti-port tube 110 that has been shaped into a serpentine form with two180° bends 126, other types and arrangements of tubes may be desirableto provide the tube run groups 104 depending on the particularrequirements for each application. By way of further example, while fivetube runs groups 104 are shown, some applications may require more thanor less than the five groups 104.

The tube run groups 104 and/or tube runs 120, 122, and 124, fins 108,inlet manifold 100, and outlet manifold 112 maybe manufactured from anysuitable material. For example in a preferred embodiment, all of thesecomponents are manufactured from aluminum and brazed together usingsuitable brazing techniques. Aluminum is one preferable material becauseit is lightweight and has a high thermal conductivity for heat transferbetween fluids. Additionally, aluminum is corrosion resistant andcapable of handling thermal stresses. Examples of other materialsinclude stainless steel, titanium, copper, and other materials suitablefor use in heat exchangers. It should be understood that all of thecomponents need not be made of the same materials. For example, thetubes 110 and fins 108 may be made out of aluminum while the inletmanifold 100 and outlet manifold 112 may be made of a material with alower thermal conductivity.

As previously discussed, the reformate flow 14 must be cooled tospecific temperature ranges prior to entering the selective oxidizers 30and 31. This is because the catalyst utilized in the selective oxidizers30 and 31 is optimized to remove CO from the reformate flow 14 atspecified temperature ranges. If the reformate flow 14 is not within thespecified temperature range, CO will not be removed in sufficientquantities for the fuel cell or will be removed inefficiently causing ashortened life for the catalyst. Additionally, if water in the reformateflow 14 were to condense, the condensed water could deactivate thecatalyst and/or shorten the catalyst life.

One option to make the reformate flow 14 temperature more uniform in anautomotive-style heat exchanger is by ensuring that the water flow 53exits the heat exchanger 10 at less than 100% vapor quality. This avoidscreating a superheated region within the heat exchanger 10, and willprevent the high reformate temperatures which are caused by therelatively low heat transfer coefficients inherent in single-phase vaporflow within the tube run groups 104. While this operation requires anadditional heat exchanger 26 to complete vaporization, such a heatexchanger would most likely be necessary to superheat the steam prior toentering the reformer 18. While this would eliminate one cause of thetemperature maldistribution, it would not eliminate the increasedreformate temperatures in the subcooled region near the inlet side 56relative to the two-phase region across the remainder of the heatexchanger 10.

More specifically, conventional automotive-style heat exchangers such asparallel flow single-pass heat exchangers (not shown), typically createa temperature maldistribution in the width direction across the outletface of the heat exchanger from the coolant inlet side to the coolantoutlet side, as shown in FIG. 4 by line E, which illustrates a model ofthe temperature distribution of the reformate flow 14 across thereformate outlet face of a conventional automotive- style, parallelflow, single-pass heat exchanger (not shown) where 0% represents theinlet side (corresponding to inlet side 56) and 100% represents theoutlet side (corresponding to outlet side 60). As illustrated, theminimum temperature T_(E)(min) of the reformate flow 14 is near 115° C.at the inlet side 56 (due to the cold temperature of the incoming waterflow 53) and the maximum temperature T_(E)(max) of the reformate flow 14is near 156° C. at a distance approximately 10% the distance from theinlet side 56 to the outlet side 60 (due to the relatively poor heattransfer coefficient inside the tubes for liquid water). After themaximum temperature peaks, the water flow 53 in the tubes begins tovaporize and the heat transfer coefficient increases thereby cooling thereformate flow 14 to the outlet temperature T_(O). For mostapplications, this temperature range is too broad for optimal CO removalin selective oxidizers 30 and 31.

By multi-passing the water flow 53 through each of the tube run groups104 and by the arrangement of the tube runs 120, 122, and 124 relativeto each other, the heat exchanger 10 and method of the present inventioncreate a reformate flow 14 with a more uniform temperature distributionwhen exiting the heat exchanger 10. As illustrated in FIG. 4, line Frepresents the temperature distribution of the reformate flow 14 acrossthe outlet face 52 of the heat exchanger 10 in the width direction fromthe inlet side 56 to the outlet side 60. The minimum temperatureT_(F)(min) of the reformate flow 14 is near 130° C. at the inlet side127 and the maximum temperature T_(F)(max) of the reformate flow 14 isnear 143° C. at a distance approximately 30% the distance from the inletside 56 to the outlet side 60. Compared to line E, line F represents atemperature range reduction of approximately 70%. Therefore, thereformate flow 14 exits the heat exchanger 10 with a much narrower andmore uniform temperature distribution when compared to a conventionalautomotive-style, parallel flow, single-pass heat exchanger.

The water flow 53 is heated as it passes through the tube runs 120, 122,and 124 and is preferably only partially vaporized prior to exiting thetube 110 as a partially vaporized flow 202. It is preferred for thewater flow 53 to only be partially vaporized to avoid a superheatedregion in the tube runs 120, 122, and 124 which would worsen thetemperature distribution. Specifically, if the water flow 53 were to befully vaporized, it would become a superheated steam flow. Superheatedsteam has a relatively low heat transfer coefficient as a single-phasevapor flow in the tube 110 when compared to a two-phase liquid and vaporflow.

The heat exchanger 10 is especially effective at decreasing thetemperature maldistribution of the reformate flow 14 because of therelationship between the first tube run 120, intermediate tube run 122,and the final tube run 124 for adjacent tube run groups 104. Toalleviate the localized cool regions associated with a conventionalautomotive-style, parallel flow, single-pass construction, multiplepasses 121, 123, and 125 are utilized to provide warmer water flow 53near the inlet side 56 via the tube runs 122 and 124. The inlet portionof the tube run 120 is typically the coldest portion of each of the tuberun groups 104, so it is sandwiched between two warmer tube runs 122 and124. Specifically, for each adjacent pair of tube run groups 104, thefirst tube run 120 of tube run group 104 is sandwiched between theintermediate tube run 122 of tube run group 104 and the final tube run124 of the adjacent tube run group 104. The partially vaporized flow 53in the intermediate tube run 122 and the final tube run 124 are at ahigher temperature than the temperature of the liquid water flow 53 atthe initial portion of the first tube run 120. Therefore, the minimumtemperature T_(F)(min) created at the first side 56 by locating thefirst tube run 120 between the intermediate tube run 122 and the finaltube run 124 is much closer to the outlet temperature T_(O) of the flow53 at the second side 60 entering the manifold 112 because the flow 53in the tube runs 122 and 124 increase the temperature of the fins 108adjacent the tube run 120. Additionally, besides increasing the minimumtemperature of the reformate flow 14, the maximum temperature isdecreased because the first tube run 120 is sandwiched between theintermediate tube run 122 and the final tube run 124. The water 53 inthe intermediate tube run 122 and the final tube run 124 is partiallyvaporized and therefore these tube runs have a higher heat transfercoefficient than the first tube run 120. Thus, only one tube run 120 pergroup of three tube runs 120, 122, 124 has a low heat transfercoefficient. The combination of the heat transfer coefficients createdby the adjacent rube runs 120, 122, 124 increases the heat transfer inthe fins 108 near the inlet side 56 and causes the reformate flow 14 tospike at a maximum temperature T_(F)(max) that is approximately 13° C.lower than the maximum temperature T_(E)(max) in a conventionalautomotive-style, parallel flow, single-pass heat exchanger.

As discussed, the partially vaporized flow 202 can be recycled torecapture the heat contained therein to other parts of the fuelprocessing subsystem as illustrated in FIG. 1. The partially vaporizedflow 202 is combined with additional water from the pressurized watersource 27 and sent to the heat exchanger 26 to be transformed into steamflow 22. Steam flow 22 is utilized in the reformer 18, thus recapturingheat from the reformate flow 14 at the heat exchangers 10 and returningit to the hydrocarbon flow 16 used to make reformate flow 14.

The heat exchangers and method of the present invention are suitable forcooling a reformate flow to within a desired temperature range whilemaintaining a narrower temperature distribution across the width of theheat exchanger than for conventional automotive-style, parallel flow,single-pass heat exchangers. The narrower temperature distribution isessential to optimizing CO removal in the fuel processing subsystemprior to flowing the reformate flow to the fuel cell. Additionally,thermal efficiency of the fuel processing subsystem 12 is improved byutilizing process water to cool the reformate flow 14.

It should be understood that while the heat exchanger 10 has beendescribed herein in connection with a fuel processing subsystem 12, heatexchangers made and operated according to the present invention canprove useful in other types of systems where a relatively uniform outlettemperature is desired across the exit face of the heat exchanger.Accordingly, no limitation to use with a fuel cell system or a fuelprocessing subsystem is intended unless expressly stated in the claims.

1. A heat exchanger for transferring heat between a first fluid flow anda second fluid flow, the heat exchanger comprising: an inlet manifold;an outlet manifold; a plurality of aligned and spaced tube run groupsextending between the inlet manifold and the outlet manifold to directthe first fluid flow through the heat exchanger, each tube run grouphaving three parallel tube runs; the tube runs of each tube run groupincluding a first tube run coupled to the inlet manifold to direct thefirst fluid in a first direction, an intermediate tube run coupled tothe first tube run to receive the first fluid therefrom and direct thefirst fluid in a second direction opposite the first direction, and afinal tube run coupled to the intermediate tube run to receive the firstfluid therefrom and direct the first fluid in the first direction to theoutlet manifold, the intermediate tube run located adjacent the firsttube run and the final tube run; for each adjacent pair of tube rungroups, the final tube run of one of the tube run groups of the pairbeing located adjacent the first tube run of the other tube run group ofthe pair; and a plurality of fins extending between the adjacent tuberuns of the tube groups to direct the second fluid flow between theadjacent tube runs of the tube groups from an inlet face to an outletface of the heat exchanger.
 2. The heat exchanger of claim 1 whereineach tube run group comprises a single tube that defines the three tuberuns.
 3. The heat exchanger of claim 2 wherein each single tube isarranged in a generally serpentine configuration.
 4. The heat exchangerof claim 1 wherein the tube runs and fins have sufficient effectivenessunder normal operating conditions to partially vaporize the first fluidflow where the first fluid flow enters the heat exchanger inlet manifoldas a single-phase fluid and exits the heat exchanger outlet manifold asa two-phase fluid.
 5. The heat exchanger of claim 1 wherein the tuberuns are flattened tubes.
 6. The heat exchanger of claim 1 wherein thefins are serpentine fins.
 7. The heat exchanger of claim 1 wherein thetube runs are constructed of aluminum.
 8. A heat exchanger fortransferring heat between a first fluid flow and a second fluid flow,the heat exchanger comprising: an inlet manifold; an outlet manifold; aplurality of aligned and spaced tubes extending between the inletmanifold and the outlet manifold to direct the first fluid flow throughthe heat exchanger, each tube having three parallel tube runs; theparallel tube runs of each tube including a first tube run coupled tothe inlet manifold to direct the first fluid flow in a first direction,an intermediate tube run coupled to the first tube run to direct thefirst fluid flow in a second direction opposite the first direction, anda final tube run coupled to the intermediate tube run and the outletmanifold to direct the first fluid flow in the first direction, theintermediate tube run located adjacent the first tube run and the finaltube run; for each adjacent pair of tubes, the final tube run of one ofthe tubes of the pair being located adjacent the first tube run of theother tube of the pair; and a plurality of fins extending between theadjacent tube runs of the tubes to direct the second fluid flow betweenthe adjacent runs of the tubes from an inlet face to an outlet face ofthe heat exchanger.
 9. The heat exchanger of claim 8 wherein the tuberuns and fins have sufficient effectiveness under normal operatingconditions to partially vaporize the first fluid flow where the firstfluid flow enters the heat exchanger inlet manifold as a single-phasefluid and exits the heat exchanger outlet manifold as a two-phase fluid.10. The heat exchanger of claim 8 wherein the tubes are flattened tubes.11. The heat exchanger of claim 8 wherein the fins are serpentine fins.12. The heat exchanger of claim 8 wherein the tubes are constructed ofaluminum.
 13. The heat exchanger of claim 8 wherein each of the tubes isarranged in a generally serpentine configuration.
 14. A method oftransferring heat from a first fluid to a second fluid in a heatexchanger, the heat exchanger including a planar inlet face to receivethe second fluid, a planar outlet face opposite the inlet face toexhaust the second fluid, a first fluid inlet on a first side of thefaces, and a first fluid outlet on a second side of the faces oppositethe first side, the method comprising the steps of: flowing the firstfluid from the inlet to the outlet via a plurality of aligned multi-passflow paths; each of the flow paths including a first pass extendingbetween the first and second sides in a first flow direction transverseto the inlet and outlet faces, an intermediate pass extending betweenthe first and second sides in a flow direction opposite the firstdirection transverse to the inlet and outlet faces, and a final passextending between the first and second sides in the first flowdirection, the passes being parallel to each other; each of theintermediate passes running between the first and final passes of theassociated flow path, for each adjacent pair of flow paths; the finalpass of one of the flow paths of the pair running adjacent the firstpass of the other flow path of the pair; flowing the second fluid fromthe first face to the second face via flow paths between and transverseto the passes; and transferring heat between the first and second fluidsas the first and second fluids flow through the respective flow paths.15. The method of claim 14 wherein the first fluid comprises water. 16.The method of claim 14 further comprising the step of transferringsufficient heat to only partially vaporize the first fluid.